Resonant cavity conditioning for improved nonlinear crystal performance

Systems and methods to improve the performance of nonlinear crystals in a resonant cavity are presented. A humidified purge gas is supplied to a resonant cavity of a laser based illumination source that includes a nonlinear crystal. A small amount of water vapor is added to a clean, dry purge gas to prevent excessive drying of the nonlinear crystal during storage or operation. In some embodiments, a humidity injection system includes at least two parallel flow paths. One flow path includes a humidity injector and another does not include humidity injection. The amount of water vapor added to the purge gas flow is determined at least in part by the relative flow rate between the parallel flow paths. In some embodiments, the amount of water vapor added to the purge gas flow is regulated based on a measurement of humidity in the resonant cavity.